A process technology for oligomerizing ethylene is commercially very useful. Among alpha-olefins prepared by an oligomerization process of ethylene, alpha-olefins currently expensive in the market are 1-hexene and 1-octene, which are important commercial raw materials, mainly used as a comonomer for preparing high value-added linear low-density polyethylene (LLDPE).
However, a conventional ethylene oligomerization process technology using an aluminum or nickel catalyst typically produces alpha-olefins having a large distribution of carbon atoms depending on a Schulz-Flory or Poisson product distribution. Therefore, the conventional technique for producing a large distribution of alpha-olefins has a limitation in producing alpha-olefins having a high value in the market with high selectivity.
Accordingly, since it was found out that ethylene may be selectively trimerized or tetramerized by transition metal catalysis to prepare 1-hexene and 1-octene in a high yield, research thereon has been actively conducted. In this regard, most of the known catalyst techniques use a chromium-based transition metal catalyst. Korean Patent Laid-Open Publication No. 10-2006-0002741 disclosed that excellent ethylene trimerization activity is possible using a PNP ligand containing a nonpolar substituent at an ortho position of a phenyl ring attached to phosphorus like (o-ethylphenyl)2PN(methyl)P(o-ethylphenyl)2. In addition, International Patent Publication No. WO2004-056479 disclosed that ethylene may be tetramerized by a chromium-based catalyst containing a PNP ligand having no substituent in a phenyl ring attached to phosphorus to improve production selectivity of 1-octene, and also discloses (phenyl)2PN(isopropyl)P(phenyl)2, etc. as an example of a heteroatom ligand used in a catalyst for such tetramerization. This patent document disclosed that though a ligand complexed chromium-based catalyst containing a heteroatom such as nitrogen and phosphorus has no polar substituent in a hydrocarbyl or heterohydrocarbyl group attached to a phosphorus atom, ethylene may be tetramerized by the catalyst to produce 1-octene with selectivity of more than 70% by mass. According to another known article, it is known in the art that when a catalyst complex was previously synthesized by a ligand having a —P—N—P— type backbone having a heteroatom between two P's and a chromium precursor and used to carry out an ethylene oligomerization reaction, the result was that there was no big change of activity and selectivity from the result of injecting the ligand and the chromium precursor separately to the oligomerization reactor. (J. Am. Chem. Soc., 2004, 126, 14712)
Meanwhile, Korean Patent Laid-Open Publication No. 10-2009-0017929 (SK Holdings) disclosed that since in a P—X—P ligand, it is due to the electronic and geometric structure of a connection structure between two phosphorus (P) atoms coordinated to chromium that trimerization or tetramerization may be selectively increased in an ethylene oligomerization reaction using a chromium-based catalyst, the most appropriate geometric structure is a connection structure of S,S—(P—C(alkyl)-C(alkyl)-P—) or R,R—(P—C(alkyl)-C(alkyl)-P—). However, the ligand of P—C—C—P— structure has a limitation in activity and selectivity, when it is injected separately from a transition metal to an ethylene oligomerization reaction medium. The reason is that the backbone structure does not possess an unshared electron pair as a pure carbon atom, and a phosphine atom giving an electron to a transition metal has a limited coordination direction to the transition metal around it due to the chiral carbon in the backbone structure, and thus, there is difficulty in forming a coordination bond with the transition metal in a reaction medium. As a result, the geometric and electronic effect of the P—C—C—P backbone structure is as follows: when the transition metal precursor and the P—C—C—P backbone ligand structure are injected separately to the ethylene oligomerization reaction medium as a catalyst like the P—N—P ligand, the number of molecules of the transition metal precursor to be converted to a catalytic active site is substantially decreased unlike the P—N—P ligand, and this results in the decrease of the activity and selectivity in trimerization or tetramerization of ethylene.
Therefore, it is known from Korean Patent Laid-Open Publication No. 2010-0087913 (SK Holdings) that a chiral ligand having a —P—C—C—P— backbone structure is previously reacted with a transition metal precursor to synthesize a chiral ligand-transition metal complex in a substantially pure form, which is then injected to an ethylene oligomerization reactor, for overcoming the limitation in improving catalyst activity. As a result, it was surprisingly found that a reaction activity was increased by 10 times or more, and the produced amount of polyethylene polymer byproduct was decreased by 1/10 times or less due to high selectivity. Consequently the polymer content after reaction may be decreased to 0.1% by mass or less, and due to the low production rate of the byproduct, the commercial process for production may be very simplified, for example, the purification process may be simplified.
According to the recent catalyst trend, as the technique for producing the expensive 1-hexene or 1-octene by selective ethylene oligomerization using a chromium-based transition metal as a catalyst has been desperately needed in the market, as compared to the conventional technique for producing various α-olefins in a large broad distribution using an alkyl aluminum or nickel-based catalyst, many attempts have been made to commercialize the technique in the related industry. As an example thereof, Sasol Technology published how the reaction conditions such as the temperature and ethylene concentration was effected on the selectivity and activity of 1-hexene and 1-octene using a continuous tubular reactor (plug flow tubular reactor, PFTR) in comparison with that using a semi-batch reactor (Journal of Catalysis, 262, 83, 2009). In the view of commercialization, the result was that since the ethylene concentration is rapidly decreased in the PFTR reactor, as the position of the tube is more distant from an ethylene inlet, it is difficult to efficiently adjust the conversion rate and the selectivity.
The crucial problem to be solved for commercializing ethylene oligomerization is to minimize a polyethylene polymer which is the byproduct produced in the oligomerization reaction and get it out of the reactor. In this regard, Korean Patent Laid-Open Publication No. 2010-0087913 disclosed that in the case of using an oligomerization catalyst having high selectivity, the production rate of the polyethylene polymer byproduct is very low, i.e., 0.1% by weight or less, and thus, the produced polymer byproduct are a precipitate in the form of small particles and present as a suspension solution in the reaction medium, thereby easily releasing the byproduct out of reactor without fouling or plugging. However, according to the published document, when the temperature is abruptly lowered in the reaction process, serious fouling may occur. U.S. Patent Application Publication No. 2012-0142089 disclosed that when removing the heat energy arising from the reaction by a cooling coil on the reactor surface in the reaction process, a heating coil is further attached so that there is no drastic temperature drop, thereby preventing from fouling. However, the cooling coil coexists with the direct heating coil on the reactor surface, so that the reaction mixture in the reactor is in direct contact with the surface of the cooling coil and heating coil, thereby having a limitation in overcoming the partly drastic temperature drop and rise of the reaction mixture.